With a view to improving the growing properties of sandy soils in desert regions it is well known to treat such soils with materials which act to limit water loss and also surface erosion. Known techniques include the incorporation of polystyrene particles in the soil and the application of an oil coating to the surface. However, such known techniques are not always wholly satisfactory in that they may be expensive and inconvenient to apply having regard to the large quantity of material required to produce an appreciable improvement, and because the resulting texture and composition of the growing medium may not be best suited to healthy, strong, plant growth.